This application is based on Japanese Patent Application No. 2001-84900, the content of which is incorporated herein by reference.
The present invention relates to an image forming apparatus provided with a transfer belt, which receives an image directly transferred on its surface or a recording medium on the surface of which an image is to be transferred.
Conventionally, in an image forming apparatus of an electrophotographic system, it has become a mainstream to employ a resin hard belt of excellent electrical and mechanical characteristics as a transfer belt. Such a transfer belt is normally rotatively driven while being stretched between a driving roller and a follower roller. It is often the case where a special tension roller, to which a spring load is applied to give tension to the transfer belt, is employed in addition to the driving roller and the follower roller.
On the other hand, as another method of giving tension to the transfer belt without employing any special tension roller, there are the method of giving tension to the belt by imposing a spring load on a movably supported follower roller, the method of providing the transfer belt constructed of a rubber belt and giving tension to the belt by the flexibility of the belt itself.
However, the resin hard belt is appropriate for the transfer belt, and if proper tension can be given to the belt providing neither a special tension roller nor a spring load operation mechanism even when this resin hard belt is employed, then it is preferable in simplifying the mechanism and reducing the cost thereof.
Accordingly, the image forming apparatus of the present invention comprises:
a driving roller;
a follower roller whose outer peripheral portion is formed of an elastic material; and
a transfer belt, which is made of a resin and stretched between the driving roller and the follower roller.
According to this construction, tension is given to the transfer belt by the elasticity of the elastic material itself of the follower roller. Accordingly, there can be provided an image forming apparatus, which needs no special tension roller and has a small-sized inexpensive transfer belt unit. Furthermore, because the elasticity is possessed not by the driving roller but by the follower roller, the transfer belt can be rotatively driven at a stabilized velocity by the driving roller having no elasticity, and a satisfactory image, which has neither color drift nor magnification shift, can be formed.
It is acceptable that the image forming apparatus of the present invention comprises a transfer belt unit constructed of the driving roller, the follower roller, the transfer belt and a frame for supporting the driving roller and the follower roller, the transfer belt unit is provided while being able to be mounted on and demounted from the image forming apparatus, and the frame is provided while being able to bend in the middle.
According to this construction, the distance between the driving roller and the follower roller can be expanded and contracted by the bend in the middle of the frame, and the mounting and demounting of the transfer belt on and from the rollers can easily be performed.
Moreover, it is acceptable that an image forming apparatus of the present invention comprises a transfer belt unit constructed of the driving roller, the follower roller, the transfer belt and a frame for supporting the driving roller and the follower roller, the transfer belt unit is provided while being able to be mounted on and demounted from the image forming apparatus, and the frame is provided while being able to expand and contract.
According to this construction, the distance between the driving roller and the follower roller can be expanded and contracted by the expansion and contraction of the frame, and the transfer belt can easily be mounted on and demounted from the rollers.
Moreover, it is preferable that the elastic material of the follower roller has an Asker-C hardness of not greater than 35 degrees.
According to this construction, optimum tension can be given to the transfer belt.
Moreover, it is acceptable that the follower roller has varied outside diameters in the center portion and the end portions thereof.
According to this construction, the transfer belt can be prevented from meandering.
Moreover, it is acceptable that the transfer belt is an intermediate transfer belt, and a transfer member for transferring an image carried on the intermediate transfer belt onto a recording medium is arranged on the driving roller side.
According to this construction, by providing the transfer member not on the follower roller side that the elastic deformation is caused by the tension of the transfer belt but on the driving roller side that such a deformation does not occur, stabilized image transfer can be performed.
Moreover, the image forming apparatus of the present invention is permitted to be provided with a driving roller, a follower roller whose outer peripheral portion is formed of an elastic material and a transfer belt that is made of a resin and stretched between the driving roller and the follower roller and given tension only by the elasticity of the follower roller. In this case, the transfer belt may be supported only by the two rollers of the driving roller and the follower roller.
Also, with this construction, tension is given to the transfer belt by the elasticity of the elastic material of the follower roller, and therefore, no special tension roller is required to be provided, allowing an image forming apparatus having a small-sized inexpensive transfer belt unit to be provided. Furthermore, because the elasticity is possessed not by the driving roller but by the follower roller, the transfer belt can be rotatively driven at a stabilized velocity by the driving roller having no elasticity, and a satisfactory image, which has neither color drift nor magnification shift, can be formed.